The present disclosure is directed to an accelerated weathering test apparatus of the type used to concentrate solar radiation on test specimens, and more particularly, to an accelerated weathering test apparatus with a fixture that facilitates a reduction in the operating temperature of the test specimens.
Manufacturers of exterior coatings, such as paints and finishes, as well as plastics and other components which tend to degrade under exposure to solar radiation and other weathering effects, often want to know how such products will perform following years of exposure. However, such manufacturers typically require such information in a much shorter time than it would take to expose such materials to weathering effects under normal conditions. Accordingly, accelerated weathering test devices have been developed which accelerate the effects of weathering due to outdoor exposure in a much shorter time so that manufacturers need not actually wait five or ten years in order to determine how their products will hold up after five or ten years of actual outdoor exposure.
One known accelerated weathering test device is disclosed in U.S. Pat. No. 2,945,417, issued to Caryl et al. The aforementioned test device includes a Fresnel-reflecting solar concentrator having a series of ten flat mirrors which focus natural sunlight onto a series of test specimens secured to a target board measuring approximately five (5) inches wide by fifty-five (55) inches long. The Fresnel-reflecting solar concentrator directs solar radiation onto the target board area with an intensity of approximately eight suns. Both the bed which supports the mirrors of the solar concentrator, and the target board, are supported by a frame which can be rotated to follow daily movements of the sun.
A solar tracking mechanism responsive to the position of the sun, controls the operation of an electric motor that is used to rotate the test apparatus to follow movements of the sun. The axis of rotation of the test machine is oriented in a north-south direction, with the north elevation having altitude adjustment capability to account for variation in the sun's altitude at various times during the year.
Such known testing devices are also provided with an air tunnel mounted above the target board. An air deflector causes air escaping from the air tunnel to be circulated across the test specimens mounted to the target board to prevent the test specimens from overheating due to the concentrated solar radiation to which they are exposed. The amount of air is controlled by the dimension of the gap between the deflector and the specimen. A squirrel cage blower communicates with the air tunnel for blowing cooling ambient air there through. In addition, water spray nozzles are provided proximate to target board for wetting the test samples at periodic intervals to simulate the weathering effects of humidity, dew, rain, etc.
Another known accelerated weathering test device is disclosed in U.S. Pat. No. 4,807,247. issued to Robins. The aforementioned test device includes all the structure previously described above with respect to the '417 patent and further includes a system for maintaining a uniform, constant test specimen temperature during daylight hours despite variations in ambient air temperature and variations in solar radiation intensity.
The system includes a temperature sensor mounted to the target board for exposure to the concentrated solar radiation and for generating an electrical signal indicative of the temperature of the test specimen mounted to the target board. The system further includes a control mechanism electrically coupled to the temperature sensor and responsive to the electrical signal generated thereby for selectively controlling the application of electrical power to the electrical motor included within the air circulation system. In this manner, the control mechanism serves to vary the speed of the electric motor and thereby control the flow rate of cooling ambient air circulating across the target board so that the temperature of the test specimen remains constant at the desired set point.
When the sensed temperature of the test specimen increases, the control mechanism increases the speed of the blower motor to circulate more cooling ambient air across the target board in order to lower the temperature of the test samples back to the desired set point. Similarly, if the sensed temperature of the target samples drops below the desired nominal temperature, the control mechanism decreases the speed of the blower to permit the test samples to warm up back to the desired set point.
The temperature control mechanism also includes a user operable adjustment mechanism, in the form of the control knob, for allowing a user to set a static, desired target specimen temperature. A bypass switch is also provided for allowing the user to operate the test device in the controlled temperature-mode as described above, or in an uncontrolled mode wherein the blower motor operates at a constant speed.
Standardized testing methods have been developed for operating accelerated weathering test devices of the type described above. The American Society for Testing and Materials (ASTM) has issued standards G90, E838, D4141, D3105, D3841, D5105, E1596 and D4364 covering the testing procedures and the operating parameters for conducting such outdoor accelerated weathering tests. Other standards and appraisals have also been developed and specified by the Society of Automotive Engineers (SAE), Ford, International Standards Organization (ISO), American National Standards Institute (ANSI), Japan Industrial Standard (JIS), namely, SAE J576, SAE J1961, Ford EJB-M1J14-A, Ford EST-M5P11-A, ISO 877, ANSI/NSF 54, JIS Z 2381 and MIL-T-22085D.
Apart from outdoor accelerated weathering test devices of the type described above, other test devices are also known which utilize an artificial source of radiation to expose the test specimens. An example of such a test device is disclosed in U.S. Pat. No. 3,664,188 issued to Kockott. While such test devices have the advantage of permitting precise control over radiation intensity, temperature and humidity, such test devices have often failed to duplicate the actual light spectrum of natural sunlight to which the specimens under test will actually be exposed in everyday use. It has been acknowledged and recognized by those of skill in the art that natural sunlight and artificial sunlight test apparatus are distinct from one another and provide different sets of empirical data.
Outdoor accelerated weathering test devices of the type described above in regard to U.S. Pat. Nos. 2,945,417 and 4,807,247, have the advantage of using natural sunlight, and hence the specimens under test are exposed to the actual spectrum of sunlight. However, disadvantages of outdoor accelerated weathering test devices have been discovered.
One such disadvantage is that the temperature of test specimens cannot be precisely controlled due to exposure to outdoor conditions. For example, exposure to multiples of incident sunlight can raise the temperature of a specimen beyond an acceptable testing range. Furthermore, the temperature of the specimen may rise high enough to damage the test specimen. To solve this problem, accelerated weathering test devices have been devised that attempt to cool the test specimen by artificially blowing air over the test specimen. However, when the outside conditions are windy, the disturbance from the wind degrades the cooling effects of the artificially blown air over the test specimen.
Therefore, there exists a need in the art for a device and a method of controlling the temperature and temperature fluctuations of a test specimen in an outdoor accelerated weathering test apparatus regardless of ambient wind disturbances.